1. Field of the Invention
This invention relates generally to a method and apparatus for muscle stimulation in the field of Functional Electrical Stimulation (FES) employing one or more adjustable surface area, electrode intramuscular lead. Potential appications are: scoliosis correction, upper and lower limb stimulation of hemi, para and quadriplegics, foot drop, respiratory support by diaphragm pacing and cardiac assistance. The particular application is reconstructive cardiac surgery for augmentation of cardiac output, and more particularly to such a method or apparatus wherein a muscle surgically implaced about the heart is electrically stimulated in synchrony with detected natural heartbeats to contract about the chambers of the heart and increase cardiac output.
2. Related Art
In recent years, a considerable amount of interest has been generated by the widespread use of cardiac stimulators and other muscle stimulators in the body to counter the effects of disease or injury which have left such muscles unable to perform satisfactorily. FES, e.g. cardiac pacing to increase cardiac output through the substitution of a higher, but physiological, heartbeat rate for that provided by the diseased or damaged heart itself, is widely known. Cardiac pacing cannot in itself replace or cure a failing heart, and for that reason natural heart transplants and artificial heart implants have become increasingly common.
In an effort to avoid the limitations of these procedures, several investigators have considered the possibility of increasing cardiac output of the patient's own failing heart by substituting body muscle for myocardial tissue and to attempt to train that muscle to operate in the same way as myocardial tissue.
The paper entitled "Functional Adaptation in Skeletal Muscle" by S. Salmons (in Trends in Neuroscience, 3:134-137 1980) describes the physiological differences between untrained and trained skeletal muscle.
In the paper entitled "Experimental Cardioplasty Using the Latissimus Dorsi Muscle Flap" by Juan C. Chachques, M.D. et al (in the Journal of Cardiovascular Surgery, 26 (5):457-462, 1985), the authors describe the procedure by which a pedicled Latissimus Dorsi flap is grafted directly to the heart in order to replace diseased or injured myocardial tissue. In the later paper entitled, "Development of a Non-tiring Stimulation of the Latissimus Dorsi Flap as a Myocardial Substitute" by J. C. Chachques et al, in Artificial Organs, 8:379-380, 1984, the authors describe the application of stimulating energy to the pedicled Latissimus Dorsi flap to train the muscle to contract at a frequency up to 80 beats per minute without encountering fatigue. Such a suitable trained muscle could be substituted for myocardial tissue.
The paper entitled "Synchronously Stimulated Skeletal Muscle Graft for Myocardial Repair" (in The Journal of Thoracic and Cardiovascular Surgery, 87 (3):325-331, 1984) describes an external system for training and stimulating a skeletal muscle cardiac graft in 1:1, 1:2 or 1:3 synchrony to detected R waves. The co-pending Medtronic U.S. patent application Ser. No. 761,252 filed Aug. 1, 1985 describes a skeletal muscle--aortic balloon system for augmenting cardiac output in synchrony with detected R waves.
In the Medtronic U.S. Pat. No. 4,411,268, a pulse generator is described for providing both conditioning and stimulating pulses to a cardiac muscle graft in synchrony with either the delivery of a stimulating pulse to the heart itself by an artificial cardiac pacemaker or in response to the sensing of a natural underlying heartbeat within the heart.
These prior systems would employ sensing in pacing electrodes within the heart and undefined electrodes applied to or extending through the muscle tissue surgically implaced around the heart or the balloon. The prior art does not appear to disclose a suitable electrode configuration for applying the muscle stimulator output pulses to the muscle itself.